Lithium secondary battery and control system therefor, and method for detecting state of lithium secondary battery

ABSTRACT

There is provided a control system for a lithium secondary battery that can quantitatively sense a deterioration state inherent in a lithium secondary battery using silicon oxide as a negative electrode active material, that is, the nonuniform reaction state of a negative electrode. A control system for a lithium secondary battery including a positive electrode, a negative electrode using silicon oxide as a negative electrode active material, and a lithium reference electrode having a reference potential with respect to the negative electrode includes measurement means for measuring a voltage V of the negative electrode with respect to the lithium reference electrode and a discharge capacity Q of the lithium secondary battery during discharge of the lithium secondary battery; generation means for generating a V-dQ/dV curve representing a relationship between dQ/dV, which is a proportion of an amount of change dQ in the discharge capacity Q to an amount of change dV in the voltage V, and the voltage V; calculation means for calculating an intensity ratio of two peaks appearing on the V-dQ/dV curve for two voltage values in the voltage V; and sensing means for sensing a state of the negative electrode utilizing the intensity ratio.

TECHNICAL FIELD

This exemplary embodiment relates to a lithium secondary batteryincluding a negative electrode using silicon oxide as a negativeelectrode active material, and a control system therefor, and a methodfor detecting the state of a lithium secondary battery.

BACKGROUND ART

As a system that detects the state of a secondary battery and controlsthe state, various ones have been proposed so far. Patent Literatures 1to 3 disclose systems that sense the state of charge (the amount ofstorage or SOC) of a secondary battery based on the battery voltage ofthe secondary battery. Patent Literature 4 discloses a system thatincludes dV/dQ calculation means for calculating the value of dV/dQ,which is the proportion of the amount of change dV in the batteryvoltage V of a secondary battery to the amount of change dQ in theamount of storage Q, and senses the state of a secondary battery systemutilizing a characteristic point appearing on a Q-dV/dQ curverepresenting the relationship between the value of the amount of storageQ and the value of dV/dQ, or a characteristic point appearing on aV-dV/dQ curve representing the relationship between the value of thebattery voltage V and the value of dV/dQ.

On the other hand, Patent Literature 5 discloses a lithium secondarybattery including a negative electrode using silicon oxide as a negativeelectrode active material.

CITATION LIST Patent Literature

-   Patent Literature 1: JP2007-292778A-   Patent Literature 2: JP11-346444A-   Patent Literature 3: JP7-294611A-   Patent Literature 4: JP2009-252381A-   Patent Literature 5: JP2997741B

SUMMARY OF INVENTION Technical Problem

The lithium secondary battery disclosed in Patent Literature 5, that is,the lithium secondary battery including a negative electrode usingsilicon oxide as a negative electrode active material, has the propertythat the lithium ion conductivity of the silicon oxide used for thenegative electrode increases as the amount of doped lithium increases.Therefore, a problem is that the amount of lithium doping accompanyingcharge and discharge is likely to be locally unbalanced in the negativeelectrode. When charge and discharge are repeated with the amount oflithium doping locally unbalanced in the negative electrode, volumechange accompanying charge and discharge increases only in parts inwhich the amount of lithium doping is larger than that of other parts inthe negative electrode, and therefore, finally, the parts may peel offthe current collector to decrease battery capacity. Therefore, in thelithium secondary battery using silicon oxide as the negative electrodeactive material, it is necessary to quantitatively sense its inherentdeterioration state, that is, the nonuniform reaction state of thenegative electrode.

But, in the techniques disclosed in Patent Literatures 1 to 3, thedeterioration state of the secondary battery (a decrease in batterycapacity or an increase in internal resistance) can be sensed, but inthese techniques, information regarding local reaction unbalance withinthe negative electrode cannot be obtained because the deteriorationstate is determined by measuring battery voltage. In addition, in thetechnique disclosed in Patent Literature 4, the point in the amount ofelectricity at which phase transition accompanied by a minute change inthe voltage of the electrode active material occurs can be determined,but the ratio of respective phases at the completion of charge cannot bequantitatively estimated. Therefore, a problem is that it is notpossible to quantitatively sense the nonuniform reaction state of thenegative electrode of the lithium secondary battery using silicon oxideas the negative electrode active material, that is, sense in whatproportion parts in which the lithium concentration is high and parts inwhich the lithium concentration is low are present.

This exemplary embodiment provides a control system for a lithiumsecondary battery that can solve the above-described problems.

Solution to Problem

A control system for a lithium secondary battery according to thisexemplary embodiment is a control system for a lithium secondary batteryincluding a positive electrode, a negative electrode using silicon oxideas a negative electrode active material, and means for obtaining apotential of the negative electrode with respect to a lithium referenceelectrode, the control system including measurement means for measuringa voltage V of the negative electrode with respect to the lithiumreference electrode and a discharge capacity Q of the lithium secondarybattery during discharge of the lithium secondary battery; generationmeans for generating a V-dQ/dV curve representing a relationship betweendQ/dV, which is a proportion of an amount of change dQ in the dischargecapacity Q to an amount of change dV in the voltage V, and the voltageV; calculation means for calculating an intensity ratio of two peaksappearing on the V-dQ/dV curve for two voltage values in the voltage V;and sensing means for sensing a state of the negative electrodeutilizing the intensity ratio.

A lithium secondary battery according to this exemplary embodiment is alithium secondary battery including a positive electrode, a negativeelectrode using silicon oxide as a negative electrode active material,and a lithium reference electrode having a reference potential withrespect to the negative electrode, including a charge and dischargecontrol portion for repeatedly charging and discharging the lithiumsecondary battery; a measurement portion for measuring a voltage V ofthe negative electrode with respect to the lithium reference electrodeand a discharge capacity Q of the lithium secondary battery duringdischarge of the lithium secondary battery; a generation portion forgenerating a V-dQ/dV curve representing a relationship between dQ/dV,which is a proportion of an amount of change dQ in the dischargecapacity Q to an amount of change dV in the voltage V, and the voltageV; a peak intensity ratio calculation portion for calculating anintensity ratio of two peaks appearing on the V-dQ/dV curve for twovoltage values in the voltage V; a peak intensity ratio sensing portionfor sensing a state of the negative electrode utilizing the intensityratio; and an information transmission portion for transmitting, whenthe sensing portion senses that a difference between the intensity ratioof the two peaks appearing on the V-dQ/dV curve for the two voltagevalues in the voltage V becomes equal to or more than a predeterminedthreshold, the information to the charge and discharge control portion,wherein the charge and discharge control portion receiving thetransmission executes means for improving a degree of uniformity oflithium concentration in the negative electrode.

A method for detecting a state of a lithium secondary battery accordingto this exemplary embodiment is a method for detecting a state of alithium secondary battery including a positive electrode, a negativeelectrode using silicon oxide as a negative electrode active material,and a lithium reference electrode having a reference potential withrespect to the negative electrode, including a measurement step ofmeasuring a voltage V of the negative electrode with respect to thelithium reference electrode and a discharge capacity Q of the lithiumsecondary battery during discharge of the lithium secondary battery; ageneration step of generating a V-dQ/dV curve representing arelationship between dQ/dV, which is a proportion of an amount of changedQ in the discharge capacity Q to an amount of change dV in the voltageV, and the voltage V; a calculation step of calculating an intensityratio of two peaks appearing on the V-dQ/dV curve for two voltage valuesin the voltage V; and a sensing step of sensing a state of the negativeelectrode utilizing the intensity ratio.

Advantageous Effect of Invention

According to this exemplary embodiment, it is possible to quantitativelysense a deterioration state inherent in a lithium secondary batteryusing silicon oxide as a negative electrode active material, that is,the nonuniform reaction state of a negative electrode.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a control system for a lithium secondarybattery according to a second exemplary embodiment.

FIG. 2 is a diagram showing V-dQ/dV curves.

FIG. 3 is a diagram showing V-dQ/dV curves before and after theexecution of a regeneration mode.

FIG. 4 is a diagram showing the relationship between capacity and thenumber of cycles for lithium secondary batteries.

FIG. 5 is a diagram showing a control system for a lithium secondarybattery according to a first exemplary embodiment.

DESCRIPTION OF EMBODIMENTS First Exemplary Embodiment

A first exemplary embodiment will be described below with reference tothe drawings.

FIG. 5 is a block diagram showing a control system for a lithiumsecondary battery according to the first exemplary embodiment.

In FIG. 5, a control system for a lithium secondary battery 1 includes alithium secondary battery 2, a charge and discharge control portion 3, ameasurement portion 4, a generation portion 5, a peak intensity ratiocalculation portion 6, and a peak intensity ratio comparison portion 7.In the control system for a lithium secondary battery 1 according to thefirst exemplary embodiment, the measurement portion 4, the generationportion 5, the peak intensity ratio calculation portion 6, and the peakintensity ratio comparison portion 7 are essential structures, and thelithium secondary battery 2 and the charge and discharge control portion3 are optional structures.

The lithium secondary battery 2 includes a positive electrode 21, anegative electrode 22, and a metal lithium reference electrode 23. Forthe negative electrode 22, silicon oxide is used as a negative electrodeactive material. The metal lithium reference electrode 23 is one ofmeans for obtaining the potential of the negative electrode 22 withrespect to lithium.

As the negative electrode active material of the negative electrode 22,silicon oxide described in Patent Literature 5 can be used. Examples ofsilicon oxide include SiO_(y)(0<y<2), SiLi_(x)O_(y)(x>0 and 2>y>0),silicates, and compounds in which slight amounts of metal elements ornonmetal elements are added to these silicon oxides. In addition, thesesilicon oxides may be crystalline or amorphous. Only one of these may beused, or two or more of these may be used in combination.

For all structures other than the negative electrode 22, for example,the positive electrode 21, the electrolytic solution, and the separator,of the structures of the lithium secondary battery 2, those used inpublicly known lithium secondary batteries can be used.

Examples of the positive electrode active material of the positiveelectrode 21 include lithium manganate having a layered structure orlithium manganate having a spinel structure, such as LiMnO₂ orLi_(x)Mn₂O₄ (0<x<2), LiCoO₂, LiNiO₂, or compounds in which some of thesetransition metals are replaced by other metals. In addition, LiFePO₄having an olivine type crystal structure can also be used. One of thesepositive electrode active materials can be used alone, or two or more ofthese positive electrode active materials can be used in combination.

The electrolytic solution material is not particularly limited as longas it is stable at the redox potential of metal lithium, and publiclyknown nonaqueous electrolytic solutions can be used. An electrolyticsolution in which an electrolyte salt is dissolved in a solvent is mostpreferred.

As the solvent, those in which two or more of cyclic carbonates, such aspropylene carbonate, ethylene carbonate, butylene carbonate, andvinylene carbonate, chain carbonates, such as dimethyl carbonate,diethyl carbonate, ethyl methyl carbonate, and dipropyl carbonate, andlactones, such as γ-butyrolactone, are mixed are preferred for thereason that they are stable at the redox potential of metal lithium.

Examples of the electrolyte salt include lithium salts, such as LiPF₆,LiAsF₆, LiAlCl₄, LiClO₄, LiBF₄, LiSbF₆, LiCF₃SO₃, LiCF₃CO₂, Li(CF₃SO₂)₂,and LiN(CF₃SO₂)₂. Only one of these electrolyte salts can be used, ortwo or more of these electrolyte salts can be used. As otherelectrolytic solutions, ionic liquids, such as quaternary ammonium-imidesalts, can be used.

In addition, not only liquid electrolytic solutions, but also gelelectrolytes in which polymers, such as polyacrylonitrile andpolyacrylate, are impregnated with the above electrolytic solutions, andsolid electrolytes, such as LiPON, Li₂S—LiP_(x)O_(y) (x=1 to 2 and y=2to 4), can also be used.

The separator is not particularly limited, and those publicly known canbe used. As the separator, porous films and nonwoven fabrics ofpolypropylene, polyethylene, or the like can be used.

Here, the characteristics of the lithium secondary battery 2 in whichsilicon oxide is used as the negative electrode active material of thenegative electrode 22 will be described.

The present inventors have found that in the lithium secondary battery 2including the negative electrode 22 using silicon oxide as the negativeelectrode active material, when the charge current is sufficiently small(for example, 0.02 C), peaks appear at 300 mV or near 300 mV(approximately 0.3 V) and at 500 mV or near 500 mV (approximately 0.5V), which are the redox potentials of silicon oxide, on a V-dQ/dV curverepresenting the relationship between dQ/dV, which is the proportion ofthe amount of change dQ in the amount of discharge Q of the lithiumsecondary battery 2 to the amount of change dV in the voltage V of thenegative electrode 22 with respect to the metal lithium referenceelectrode 23 during discharge, and the value of the voltage V of thenegative electrode 22 with respect to the metal lithium referenceelectrode 23, and the intensity ratio of the peaks changes with chargecapacity per silicon oxide in the negative electrode 22. Here, 0.02 Cmeans a current with such a magnitude that when the lithium secondarybattery 2 is charged with a constant current of 0.02 C, the charge ofthe lithium secondary battery 2 is completed in 50 hours. In addition,approximately 0.3 V and approximately 0.5 V indicate being in the rangeof ±10% from 0.3 V and 0.5 V, respectively.

FIG. 2 is a diagram showing V-dQ/dV curves during discharge when SiO isused as the silicon oxide. The charge current is 0.02 C.

The present inventors have found that, as shown in FIG. 2, when thecharge capacity (the amount of lithium doping) is sufficiently small(for example, when the charge capacity is 1750 mAh/g in FIG. 2), only apeak at approximately 0.5 V appears on the V-dQ/dV curve, and the peakat approximately 0.5 V increases as the charge capacity increases.Further, the present inventors have found that when the charge capacityexceeds a certain value, the peak intensity at approximately 0.5 V isconstant, and the second peak appears at approximately 0.3 V on theV-dQ/dV curve, and the peak at approximately 0.3 V increases as thecharge capacity increases. This is considered to be because when theamount of lithium doping in the silicon oxide exceeds a certain value,the second phase with different redox potential occurs in the siliconoxide. The amount of lithium contained in the phase having a peak atapproximately 0.5 V is smaller than that in the phase having a peak atapproximately 0.3 V.

In other words, the present inventors have found that from the intensityratio of these two peaks, information regarding the proportion of partsin which the lithium content is high to parts in which the lithiumcontent is low in the negative electrode 22 can be obtained.

In a lithium secondary battery including a negative electrode in whichsilicon oxide is used as a negative electrode active material, theintensity ratio of these two peaks may be different even in the sameamount of charge due to differences in various conditions. For example,when a charge and discharge cycle is repeated, the charge and dischargereactions of the negative electrode gradually become nonuniform, andmany phases in which the lithium content is high may occur. This isbecause the lithium ion conductivity of the silicon oxide changeslargely depending on the amount of contained lithium, and as the amountof contained lithium increases, the lithium ion conductivity increases.Therefore, a charge reaction is likely to occur in parts in which theamount of contained lithium is large, and as a result, the lithiumcontent in the parts is likely to increase further after charge.

The control system for a lithium ion secondary battery 1 according tothis exemplary embodiment quantifies and senses the uniformity oflithium concentration, that is, the uniformity of the state of charge,in the negative electrode 22 of lithium ion secondary battery 2utilizing the intensity ratio of these peaks. Thus, handling, such asstopping the operation of the lithium secondary battery, can beperformed.

The charge and discharge control portion 3 shown in FIG. 5 can generallybe referred to as charge and discharge control means. The charge anddischarge control portion 3 repeatedly charges and discharges thelithium secondary battery 2.

The measurement portion 4 can generally be referred to as measurementmeans. The measurement portion 4 measures the voltage V of the negativeelectrode 22 with respect to the metal lithium reference electrode 23and the discharge capacity Q of the lithium secondary battery 2 duringthe discharge of the lithium secondary battery 2. For example, themeasurement portion 4 measures the voltage V and the discharge capacityQ during first discharge and during second discharge performed after thefirst discharge respectively. The measurement portion 4 includes avoltage detection portion 41, a current detection portion 42, and adischarge capacity calculation portion 43.

The voltage detection portion 41 detects the voltage V of the negativeelectrode 22 with respect to the metal lithium reference electrode 23during each discharge (at least the first discharge and the seconddischarge) of the lithium secondary battery 2. The voltage detectionportion 41 outputs the value of the voltage V to the generation portion5.

The current detection portion 42 detects current I flowing from thelithium secondary battery 2 during each discharge (at least the firstdischarge and the second discharge) of the lithium secondary battery 2.The current detection portion 42 outputs the value of the current I tothe discharge capacity calculation portion 43.

The discharge capacity calculation portion 43 calculates the dischargecapacity Q of the lithium secondary battery 2 by adding up the values ofthe current I every predetermined time T during each discharge of thelithium secondary battery 2. The discharge capacity calculation portion43 outputs the value of the discharge capacity Q to the generationportion 5.

The generation portion 5 can generally be referred to as generationmeans. The generation portion 5 generates a V-dQ/dV curve representingthe relationship between dQ/dV, which is the proportion of the amount ofchange dQ in the discharge capacity Q to the amount of change dV in thevoltage V, and the voltage V. For example, the generation portion 5generates a V-dQ/dV curve, each time the voltage V and the dischargecapacity Q are measured, based on the measured voltage V and dischargecapacity Q. The generation portion 5 outputs the V-dQ/dV curve to thepeak intensity ratio calculation portion 6.

The peak intensity ratio calculation portion 6 can generally be referredto as calculation means. The peak intensity ratio calculation portion 6calculates the intensity ratio of two peaks appearing on the V-dQ/dVcurve for two voltage values in the voltage V. For example, the peakintensity ratio calculation portion 6 calculates, each time a V-dQ/dVcurve is generated, the intensity ratio of two peaks appearing on theV-dQ/dV curve for the two voltage values. In this exemplary embodiment,as the two voltage values in the voltage V, 0.3 V and 0.5 V are used.Instead of 0.3 V, a voltage of approximately 0.3 V may be used, andinstead of 0.5 V, a voltage of approximately 0.5 V may be used. The peakintensity ratio calculation portion 6 outputs the intensity ratio to thepeak intensity ratio comparison portion 7.

The peak intensity ratio comparison portion 7 can generally be referredto as sensing means. The peak intensity ratio comparison portion 7senses the state of the negative electrode 22 utilizing the intensityratio. For example, the peak intensity ratio comparison portion 7compares intensity ratios calculated by the peak intensity ratiocalculation portion 6 during a plurality of discharges with each other,and senses the state of the negative electrode 22 from the result of thecomparison. As one example, the peak intensity ratio comparison portion7 senses that the degree of uniformity of lithium concentration in thenegative electrode 22 becomes equal to or less than a predeterminedvalue, that is, an unbalance in the amount of lithium dopingaccompanying charge and discharge occurs in the negative electrode 22and a nonuniform reaction state in the negative electrode 22 occurs,when the difference between an intensity ratio calculated based on thevoltage V and the discharge capacity Q during the second discharge andan intensity ratio calculated based on the above voltage V and dischargecapacity Q during the first discharge is equal to or more than apredetermined threshold.

Next, operation will be described.

While the charge and discharge control portion 3 allows the lithiumsecondary battery 2 to perform discharge operation, the voltagedetection portion 41 obtains the voltage V of the negative electrode 22with respect to the metal lithium reference electrode 23 by measuringthe voltage between the negative electrode 22 and the metal lithiumreference electrode 23.

Alternatively, it is also possible that the discharge curve of a halfcell comprising a positive electrode and a metal lithium negativeelectrode is previously measured, and the measurement portion 4 obtainsthe voltage V of the negative electrode 22 with respect to the metallithium reference electrode 23 by calculation from the differencebetween the discharge curve of the lithium secondary battery 2 and thedischarge curve of the half cell. Since in a positive electrode oflithium manganate having a spinel structure, LiCoO₂, LiNiO₂, LiFePO₄, orthe like currently put to practical use in a lithium secondary battery,charge and discharge reactions proceed uniformly and stably comparedwith those in a negative electrode of silicon oxide, there is nopractical problem even if the discharge curve of the positive electrodeis considered to be almost constant at any current value forconvenience.

In addition, the current detection portion 42 detects the current Iflowing from the lithium secondary battery 2 while the lithium secondarybattery 2 performs discharge operation. The discharge capacitycalculation portion 43 calculates the discharge capacity Q by adding upthe current values I detected by the current detection portion 42, everypredetermined time T. The measurement portion 4 obtains the voltage Vand the discharge capacity Q by the above methods every predeterminedtime T during the discharge of the lithium secondary battery 2.

Based on the detection results of the measurement portion 4, thegeneration portion 5 calculates the amount of change dV in the voltage Vand the amount of change dQ in the discharge capacity Q for eachpredetermined time T, and, based on these, obtains the value of dQ/dVfor each predetermined time T. The generation portion 5 draws a V-dQ/dVcurve from these values of dQ/dV and the values of the voltage V.

The peak intensity ratio calculation portion 6 obtains the intensities(integrated intensities) of peaks on the V-dQ/dV curve by approximatingthe peaks on the V-dQ/dV curve by a Gaussian function and integratingthem, and calculates the intensity ratio.

The peak intensity comparison portion 7 senses the uniformity of thenegative electrode reaction by comparing this intensity ratio with apeak intensity ratio obtained from a V-dQ/dV curve when the lithiumsecondary battery 2 is charged with sufficiently small current (forexample, 0.02 C).

Second Exemplary Embodiment

A second exemplary embodiment will be described below with reference toFIG. 1.

A control system for a lithium secondary battery 1 according to thesecond exemplary embodiment includes a lithium secondary battery 2, acharge and discharge control portion 3, a measurement portion 4, ageneration portion 5, a peak intensity ratio calculation portion 6, anda peak intensity ratio comparison portion 7, as in the first exemplaryembodiment, but is different in further including an informationtransmission portion 8. In the control system for a lithium secondarybattery 1 according to the second exemplary embodiment, the charge anddischarge control portion 3, the measurement portion 4, the generationportion 5, the peak intensity ratio calculation portion 6, the peakintensity ratio comparison portion 7, and the information transmissionportion 8 are essential structures, and the lithium secondary battery 2is an optional structure.

The information transmission portion 8 can generally be referred to asinformation transmission means, and transmits information regardingintensity ratios obtained by the peak intensity ratio comparison portion7 to the charge and discharge control portion 3.

In the second exemplary embodiment, the peak intensity ratio comparisonportion 7 compares a peak intensity ratio in an ideal uniform state withmeasured peak intensity, and when the difference between both exceeds acertain value (for example, 10% or more, hereinafter referred to as athreshold), the peak intensity ratio comparison portion 7 transmits theinformation to the charge and discharge control portion through theinformation transmission portion 8, and executes a regeneration mode(charge or discharge with microcurrent). By performing charge anddischarge with microcurrent (for example, 0.02 C), the uniformity oflithium concentration in the negative electrode can be improved.

FIG. 3 is a diagram showing V-dQ/dV curves during discharge when acharge and discharge cycle is repeated in a lithium secondary batteryincluding a negative electrode in which SiO is used as silicon oxide. Byrepeating the charge and discharge cycle (1 C current), the peakintensity ratio on the V-dQ/dV curve changes, and when a regenerationmode (0.02 C) is executed when the deviation from the reference value ofthe peak intensity ratio exceeds 10% in the 53th cycle, the deviationfrom the reference value of the peak intensity ratio returns to withinthe threshold when the cycle test is resumed after the execution. Thisindicates that for the nonuniformity of lithium concentration, theuniformity is improved by the execution of the regeneration mode.

The threshold is not particularly limited, and can be set, for example,in the range of 5 to 20%. In addition, the amount of the microcurrent isalso not particularly limited, and can be set, for example, in the rangeof 0.01 C to 0.1 C.

Next operation will be described. The operation of the lithium secondarybattery 2, the charge and discharge control portion 3, the measurementportion 4, the generation portion 5, the peak intensity ratiocalculation portion 6, and the peak intensity ratio comparison portion 7is similar to that of the first exemplary embodiment. In the secondexemplary embodiment, when the occurrence of a nonuniform reaction statein the negative electrode 22 is sensed by the peak intensity comparisonportion 7, the information transmission portion 8 transmits theinformation to the charge and discharge control portion 3, and thecharge and discharge control portion executes a regeneration mode(charge and discharge with microcurrent, for example, 0.02 C).

EXAMPLES

Specific Examples will be described below.

Example 1

<Fabrication of Negative Electrode 22>

Silicon monoxide (average particle diameter D50=25 μm) manufactured byKojundo Chemical Laboratory Co., Ltd., carbon black (trade name: #3030B,manufactured by Mitsubishi

Chemical Corporation), and polyamic acid (trade name: U-Varnish A,manufactured by Ube Industries, Ltd.) were measured at a mass ratio of83:2:15. They were mixed with n-methylpyrrolidone (NMP) using ahomogenizer to provide a slurry. The mass ratio of the NMP and thesolids was 57:43. The slurry was applied to 15 μm thick Cu0.2Sn using adoctor blade, and then heated at 120° C. for 7 minutes to dry the NMP toprovide a negative electrode 22.

Then, the negative electrode 22 was heated under a nitrogen atmosphereat 250° C. for 30 minutes using an electric furnace.

<Fabrication of Positive Electrode 21>

Lithium cobaltate manufactured by Nichia Corporation, carbon black(trade name: #3030B, manufactured by Mitsubishi Chemical Corporation),and polyvinylidene fluoride (trade name: #2400, manufactured by KUREHACORPORATION) were measured at a mass ratio of 95:2:3. They were mixedwith NMP to provide a slurry. The mass ratio of the NMP and the solidswas 52:48. The slurry was applied to 15 μm thick aluminum foil using adoctor blade, and then heated and dried at 120° C. for 5 minutes.

<Fabrication of Lithium Secondary Battery 2>

An aluminum terminal and a nickel terminal were welded to the abovepositive electrode 21 and negative electrode 22, respectively. Inaddition, a nickel terminal was welded to copper foil and lithium foilbonded together (manufactured by Honjo Metal Co., Ltd.) to provide alithium reference electrode (metal lithium reference electrode) 23.These were laminated via separators to fabricate an electrode element.The electrode element was packaged with a laminate film, and anelectrolytic solution was injected. Then, while the pressure wasreduced, the laminate film was heat-sealed for sealing to fabricate aflat plate type lithium secondary battery 2. For the separator, apolypropylene film was used. For the laminate film, analuminum-deposited polypropylene film was used. For the electrolyticsolution, a 7:3 (volume ratio) mixed solvent of ethylene carbonate anddiethyl carbonate containing 1.0 mol/l of a LiPF₆ electrolyte salt wasused.

<Evaluation of Lithium Secondary Battery 2>

The fabricated lithium secondary battery 2 was charged and discharged inthe voltage range of 4.2 V to 2.7 V using the charge and dischargecontrol portion 3 to perform a charge and discharge cycle test. Thecharge was performed by a CCCV method (constant current (1 C) to 4.2 V,and after 4.2 V was reached, the voltage was kept constant for 1 hour),and the discharge was performed by a CC method (constant current (1 C)).Here, the 1 C current means a current with such a magnitude that when abattery with any capacity is discharged with constant current, thedischarge is completed in 1 hour. In the charge and discharge cycletest, a charge and discharge tester ACD-100M (trade name) manufacturedby ASKA Electronic Co., Ltd. was used as the charge and dischargecontrol portion 3.

While the charge and discharge cycle test was performed, simultaneously,the measurement portion 4 measured the voltage V between the negativeelectrode 22 and the lithium reference electrode (metal lithiumreference electrode) 23, and calculated discharge capacity Q fromdischarge time and a discharge current value. The voltage V and thedischarge capacity Q were recorded every 10 minutes or each time achange of 0.04 V occurred in voltage. The generation portion 5 drew adischarge curve from the voltage V and the discharge capacity Q, andobtained a V-dQ/dV curve from the obtained discharge curve.

The peak intensity ratio calculation portion 6 obtained the intensity ofa peak at approximately 0.3 V and the intensity of a peak atapproximately 0.5 V on the V-dQ/dV curve by approximation by a Gaussianfunction. When the ratio of the two peak intensities changed by ±10% ormore from the initial value, the next charge and discharge cycle was setto be performed with a constant current of 0.02 C (regeneration mode).

Comparative Example 1

As Comparative Example 1, a battery fabricated as in Example 1 wassimilarly subjected to a charge and discharge cycle test except that noregeneration mode was performed.

FIG. 4 is a diagram showing the relationship between capacity and thenumber of cycles for the lithium secondary batteries 2 of Example 1 andComparative Example 1. Referring to FIG. 4, it is found that inComparative Example 1 in which no regeneration mode was performed, adecrease in capacity is seen in a smaller number of cycles than inExample 1. From FIG. 4, it is described that the present battery controlsystem can sense the state of the negative electrode 22, and execute aregeneration mode as required, to reduce a decrease in the capacity ofthe lithium secondary battery 2 accompanying charge and dischargecycles.

Example 2

In this Example, the control system for a lithium secondary battery hadstructures similar to those of Example 1, but the threshold and theamount of current in the regeneration mode were different from those ofExample 1. When the ratio of two peak intensities changed by not lessthan a threshold described in Table 1 from the initial value, the nextcharge and discharge cycle was executed with a constant current of 0.1 C(regeneration mode).

TABLE 1 Ratio of peak intensity Ratio of peak intensity ratio beforeratio after Threshold regeneration mode regeneration mode (%) to initialvalue (%) to initial value (%) Example 2-1 5 93.8 96.7 Example 2-2 1087.2 93.6 Example 2-3 15 81.5 88.8 Example 2-4 20 77.3 84.1

In addition, when the ratio of two peak intensities changed by 20% ormore from the initial value, the next charge and discharge cycle wasexecuted in the range of constant current described in Table 2.

TABLE 2 Ratio of peak Ratio of peak intensity ratio before intensityratio after Current value regeneration mode regeneration mode (C) toinitial value (%) to initial value (%) Example 2-5 0.01 77.8 98.9Example 2-6 0.02 78.2 98.6 Example 2-7 0.03 77.5 97.8 Example 2-8 0.0576.8 89.4 Example 2-9 0.1 77.3 84.1

From Examples 2-1 to 2-4, it was confirmed that in the range in whichthe threshold was 5 to 20%, the deviation from the reference value ofthe peak intensity ratio returned to within the threshold after theexecution of the regeneration mode. This is considered to be because theuniformity of lithium concentration was improved by the execution of theregeneration mode. In addition, also in the cases where the thresholdwas set to 20%, from Examples 2-5 to 2-9, it was confirmed that in thecases where the amount of current of the charge and discharge during theregeneration mode was 0.01 C to 0.1 C, the deviation from the referencevalue of the peak intensity ratio returned to within the threshold afterthe execution of the regeneration mode.

In the exemplary embodiments and the Examples described above, theillustrated structures or the calculation in the correction program ismerely one example, and this exemplary embodiment is not limitedthereto.

According to this exemplary embodiment, during the discharge of thelithium secondary battery 2 including the negative electrode 22 usingsilicon oxide as the negative electrode active material, the measurementportion 4 detects the voltage V of the negative electrode 22 withrespect to the metal lithium reference electrode 23 and the dischargecapacity Q of the lithium secondary battery 2, the generation portion 5generates a V-dQ/dV curve, the peak intensity ratio calculation portion6 calculates the intensity ratio of two peaks appearing on the V-dQ/dVcurve, and the peak intensity ratio comparison portion 7 senses thestate of the negative electrode 22 utilizing the intensity ratio.

In addition, in this exemplary embodiment, as the two voltage values,0.3 V and 0.5 V are used.

In addition, in this exemplary embodiment, the charge and dischargecontrol portion 3 repeatedly charges and discharges the lithiumsecondary battery 2, the measurement portion 4 measures the voltage Vand the discharge capacity Q during the first discharge and during thesecond discharge respectively, the generation portion 5 generates aV-dQ/dV curve, each time the voltage V and the discharge capacity Q aremeasured, based on the measured voltage V and discharge capacity Q, thepeak intensity ratio calculation portion 6 calculates, each time theV-dQ/dV curve is generated, the intensity ratio of two peaks appearingon the generated V-dQ/dV curve for the two voltage values, and the peakintensity ratio comparison portion 7 compares the intensity ratios eachcalculated during each discharge with each other, and senses the stateof the negative electrode 22 from the result of the comparison.

In addition, in this exemplary embodiment, the peak intensity ratiocomparison portion 7 senses that the degree of uniformity of lithiumconcentration in the negative electrode 22 becomes equal to or less thana predetermined value when the difference between the intensity ratiocalculated based on the voltage V and the discharge capacity Q duringthe second discharge and the intensity ratio calculated based on thevoltage V and the discharge capacity Q during the first discharge isequal to or more than a predetermined threshold (for example, ±10% inthe example shown in FIG. 4). The threshold is not limited to ±10%, andcan be appropriately changed, and may be, for example, ±20%.

The intensity ratio of two peaks appearing on the V-dQ/dV curve for thetwo voltage values changes according to the proportion of parts in whichthe lithium content is high to parts in which the lithium content is lowin the negative electrode 22.

Therefore, the peak intensity ratio comparison portion 7 can sense theuniformity of the reaction in the negative electrode 22 of the lithiumsecondary battery 2 utilizing silicon oxide for the negative electrodeactive material, with good precision. In other words, from the intensityratio of these two peaks, information regarding the proportion of partsin which the lithium content is high to parts in which the lithiumcontent is low in the negative electrode 22 can be obtained.

The charge reaction of the negative electrode 22 having silicon oxide isbasically due to a common mechanism based on a reaction in which siliconin silicon oxide and lithium form an alloy. Therefore, in all thelithium secondary batteries using silicon oxide as the active materialof the negative electrode described above, the uniformity of thenegative electrode reaction state can be quantified and sensed by thecontrol system for a lithium secondary battery 1 in this exemplaryembodiment.

This application claims priority to Japanese Patent Application No.2010-175337 filed Aug. 4, 2010, and Japanese Patent Application No.2010-287956 filed Dec. 24, 2010, the entire disclosure of which isincorporated herein.

While the invention of this application has been described withreference to the exemplary embodiments and the Examples, the inventionof this application is not limited to the above exemplary embodimentsand Examples. Various changes that can be understood by those skilled inthe art can be made in the configuration and details of the invention ofthis application within the scope of the invention of this application.

Reference Signs List

-   1 control system for lithium secondary battery-   2 lithium secondary battery-   21 positive electrode-   22 negative electrode-   23 lithium reference electrode-   3 charge and discharge control portion-   4 detection portion-   41 voltage detection portion-   42 current detection portion-   43 discharge capacity calculation portion-   5 generation portion-   6 peak intensity ratio calculation portion-   7 peak intensity ratio comparison portion-   8 information transmission portion

1. A control system for a lithium secondary battery comprising apositive electrode, a negative electrode using silicon oxide as anegative electrode active material, and means for obtaining a potentialof the negative electrode with respect to a lithium reference electrode,the control system comprising: measurement means for measuring a voltageV of the negative electrode with respect to the lithium referenceelectrode and a discharge capacity Q of the lithium secondary batteryduring discharge of the lithium secondary battery; generation means forgenerating a V-dQ/dV curve representing a relationship between dQ/dV,which is a proportion of an amount of change dQ in the dischargecapacity Q to an amount of change dV in the voltage V, and the voltageV; calculation means for calculating an intensity ratio of two peaksappearing on the V-dQ/dV curve for two voltage values in the voltage V;and sensing means for sensing a state of the negative electrodeutilizing the intensity ratio.
 2. The control system for a lithiumsecondary battery according to claim 1, wherein the two voltage valuesare redox potentials of the silicon oxide.
 3. The control system for alithium secondary battery according to claim 1, wherein the two voltagevalues are approximately 0.3 V and approximately 0.5 V.
 4. The controlsystem for a lithium secondary battery according to claim 1, furthercomprising: charge and discharge control means for repeatedly chargingand discharging the lithium secondary battery, wherein the measurementmeans measures the voltage V and the discharge capacity Q during firstdischarge and during second discharge performed after the firstdischarge respectively, the generation means generates the V-dQ/dVcurve, each time the voltage V and the discharge capacity Q aremeasured, based on the voltage V and the discharge capacity Q, thecalculation means calculates, each time the V-dQ/dV curve is generated,an intensity ratio of two peaks appearing on the V-dQ/dV curve for thetwo voltage values, and the sensing means compares the intensity ratioscalculated by the calculation means with each other, and senses thestate of the negative electrode from a result of the comparison.
 5. Thecontrol system for a lithium secondary battery according to claim 4,wherein the sensing means senses that a degree of uniformity of lithiumconcentration in the negative electrode becomes equal to or less than apredetermined value when a difference between the intensity ratiocalculated based on the voltage V and the discharge capacity Q duringthe second discharge and the intensity ratio calculated based on thevoltage V and the discharge capacity Q during the first discharge isequal to or more than a predetermined threshold.
 6. The control systemfor a lithium secondary battery according to claim 5, wherein thesensing means further comprises information transmission means fortransmitting, when the sensing means senses that the difference betweenthe intensity ratio calculated based on the voltage V and the dischargecapacity Q during the second discharge and the intensity ratiocalculated based on the voltage V and the discharge capacity Q duringthe first discharge becomes equal to or more than a predeterminedthreshold, the information to the charge and discharge control means,and the charge and discharge control means receiving the transmissionexecutes means for improving the degree of uniformity of the lithiumconcentration in the negative electrode.
 7. The control system for alithium secondary battery according to claim 5, wherein thepredetermined threshold is 5 to 20%.
 8. The control system for a lithiumsecondary battery according to claim 7, wherein the means for improvingthe degree of uniformity of the lithium concentration in the negativeelectrode is charge and discharge with microcurrent.
 9. The controlsystem for a lithium secondary battery according to claim 8, wherein anamount of the microcurrent is 0.01 C to 0.1 C.
 10. A lithium secondarybattery comprising a positive electrode, a negative electrode usingsilicon oxide as a negative electrode active material, and a lithiumreference electrode having a reference potential with respect to thenegative electrode, the lithium secondary battery comprising: a chargeand discharge control portion for repeatedly charging and dischargingthe lithium secondary battery; a measurement portion for measuring avoltage V of the negative electrode with respect to the lithiumreference electrode and a discharge capacity Q of the lithium secondarybattery during discharge of the lithium secondary battery; a generationportion for generating a V-dQ/dV curve representing a relationshipbetween dQ/dV, which is a proportion of an amount of change dQ in thedischarge capacity Q to an amount of change dV in the voltage V, and thevoltage V; a peak intensity ratio calculation portion for calculating anintensity ratio of two peaks appearing on the V-dQ/dV curve for twovoltage values in the voltage V; a peak intensity ratio comparisonportion for sensing a state of the negative electrode utilizing theintensity ratio; and an information transmission portion fortransmitting, when the comparison portion senses that a differencebetween the intensity ratio of the two peaks appearing on the V-dQ/dVcurve for the two voltage values in the voltage V becomes equal to ormore than a predetermined threshold, the information to the charge anddischarge control portion, wherein the charge and discharge controlportion receiving the transmission executes means for improving a degreeof uniformity of lithium concentration in the negative electrode. 11.The lithium secondary battery according to claim 10, wherein the twovoltage values are redox potentials of the silicon oxide.
 12. Thelithium secondary battery according to claim 10, wherein the two voltagevalues are approximately 0.3 V and approximately 0.5 V.
 13. The lithiumsecondary battery according to claim 10, wherein the measurement portionmeasures the voltage V and the discharge capacity Q during firstdischarge and during second discharge performed after the firstdischarge respectively, the generation portion generates the V-dQ/dVcurve, each time the voltage V and the discharge capacity Q aremeasured, based on the voltage V and the discharge capacity Q, the peakintensity ratio calculation portion calculates, each time the V-dQ/dVcurve is generated, an intensity ratio of two peaks appearing on theV-dQ/dV curve for the two voltage values, and the peak intensity ratiocomparison portion compares the intensity ratios calculated by thecalculation portion with each other, and senses the state of thenegative electrode from a result of the comparison.
 14. The lithiumsecondary battery according to claim 13, wherein the comparison portionsenses that the degree of uniformity of the lithium concentration in thenegative electrode becomes equal to or less than a predetermined valuewhen a difference between the intensity ratio calculated based on thevoltage V and the discharge capacity Q during the second discharge andthe intensity ratio calculated based on the voltage V and the dischargecapacity Q during the first discharge is equal to or more than apredetermined threshold.
 15. The lithium secondary battery according toclaim 14, wherein the predetermined threshold is 5 to 20%.
 16. Thelithium secondary battery according to claim 15, wherein the charge anddischarge control portion receiving the transmission performs charge anddischarge with microcurrent.
 17. The lithium secondary battery accordingto claim 16, wherein an amount of the microcurrent is 0.01 C to 0.1 C.18. A method for detecting a state of a lithium secondary batterycomprising a positive electrode, a negative electrode using siliconoxide as a negative electrode active material, and a lithium referenceelectrode having a reference potential with respect to the negativeelectrode, the method comprising: a measurement step of measuring avoltage V of the negative electrode with respect to the lithiumreference electrode and a discharge capacity Q of the lithium secondarybattery during discharge of the lithium secondary battery; a generationstep of generating a V-dQ/dV curve representing a relationship betweendQ/dV, which is a proportion of an amount of change dQ in the dischargecapacity Q to an amount of change dV in the voltage V, and the voltageV; a calculation step of calculating an intensity ratio of two peaksappearing on the V-dQ/dV curve for two voltage values in the voltage V;and a sensing step of sensing a state of the negative electrodeutilizing the intensity ratio.
 19. The method for detecting a state of alithium secondary battery according to claim 18, wherein the two voltagevalues are redox potentials of the silicon oxide.
 20. The method fordetecting a state of a lithium secondary battery according to claim 18,wherein the two voltage values are approximately 0.3 V and approximately0.5 V.